Steel pipes known as oil country tubular goods or line pipes fall into two broad categories: welded steel pipes (e.g., electric resistance welded steel pipes and UOE steel pipes) and seamless steel pipes. Of these kinds of steel pipes, electric resistance welded steel pipes can be manufactured at low cost by using a hot rolled steel strip (so-called a hot rolled steel coil) as the raw material and thus are economically advantageous.
However, electric resistance welded steel pipes are usually manufactured by forming steel strips into cylindrical open pipes with forming rolls (here, the open pipes are pipe-like steel strips that are formed using multiple forming rolls and have the edges thereof unwelded; hereinafter, such pipe-like steel strips are referred to as open pipes) and then welding the longitudinal edges of the open pipes (i.e., both edges of each cylindrical steel strip) by electric resistance welding (also referred to as high-frequency resistance welding) while compressing the longitudinal edges using squeeze rolls; thus, electrical resistance welded steel pipes unavoidably have a weld (so-called a seam) and the problem of a deteriorated low-temperature toughness of the seam. Thus, oil country tubular goods and line pipes based on electric resistance welded steel pipes have a problem with the use in cold districts. The reason why the low-temperature toughness of the seam is deteriorated is as follows: When the longitudinal edges are welded, hot molten metal reacts with oxygen in the air to form oxide, and the oxide is likely to remain in the seam.
Electric resistance welded steel pipes have another problem: Alloy elements often segregate in molten metal while the longitudinal edges are being welded, and thus the corrosion resistance of the seam is often deteriorated. Thus, oil country tubular goods and line pipes based on electric resistance welded steel pipes have a problem with the use in the harsh corrosion environment (e.g., the sour environment).
Incidentally, welding with laser beams (hereinafter, referred to as laser welding) has attracted attention as a welding method that does not deteriorate the low-temperature toughness or the corrosion resistance of the seam. With laser welding, the heat source can be small in dimensions, heat energy can converge to a high density, and thus the formation of oxide and the segregation of alloy elements in molten metal can be prevented. Therefore, if laser welding is applied to the manufacturing of welded steel pipes, then the deterioration of the low-temperature toughness and corrosion resistance of the seam can be prevented.
So, the manufacturing process of welded steel pipes has introduced a technology to produce a steel pipe by welding the longitudinal edges of an open pipe under irradiation with a laser beam (i.e., laser welded steel pipes).
In laser welding, however, molten metal is formed in a very narrow area. As a result, the seam of the laser welded steel pipe may have an opening if the contact point of the longitudinal edges of the open pipe, at which the squeeze rolls compress the longitudinal edges (hereinafter, referred to as the contact point or the squeezing point) has a shift from the circumferential position of irradiation with the laser beam; the portion having such an opening should be handled as a welding defect and removed, and this decreases the yield rate of laser welded steel pipes.
As a solution to this, researchers have examined various techniques to monitor the status of irradiation with a laser beam during the manufacturing of a laser welded steel pipe.
For example, Japanese Unexamined Patent Application Publication No. H10-76383 discloses a technique to assess the status of laser welding, in which a laser beam is applied to one side of a steel strip, and a plasma illumination emerging on the other side is monitored. However, the plasma illumination scatters over a wide area, and thus, with this technique, it is difficult to have an accurate understanding of the status of laser welding and impossible to detect any shift of the position of irradiation with the laser beam from the longitudinal edges accurately.
And, Japanese Unexamined Patent Application Publication No. H8-267241 discloses a technique to assess the formation status of penetration beads, in which the emission intensity generated by laser welding is measured. However, the emission intensity greatly varies because of various factors, and thus, with this technique, it is difficult to have an accurate understanding of the formation status of penetration beads.
Japanese Unexamined Patent Application Publication No. 2001-25867 discloses a technique to control welding conditions, in which molten metal formed by arc welding is imaged, and the obtained image is used to analyze the shape of penetration beads. The direct application of this technique for arc welding to laser welding fails to provide a clear image of molten metal. This is because in laser welding, heat energy converges to a high density, and thus an excessive light intensity is generated. As a result, it is difficult in laser welding to have an accurate understanding of the shape of penetration beads.
In addition, Japanese Unexamined Patent Application Publication No. 2001-25867 discloses a technique of irradiating molten metal with a laser beam through an interference filter; however, this laser beam is used to image molten metal and thus has no contribution to welding.
The present invention is intended to provide a method for manufacturing a laser welded steel pipe at a high yield rate and in a stable manner, in which the status of laser welding is accurately assessed during manufacturing of the laser welded steel pipe, and the assessment is used to modify welding conditions.